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A recent test surprised transit officials showing compressed natural gas buses not only work at altitude, they may be a little stronger on high mountain passes than the existing diesel fleet.

But switching to the clean-fuel coaches is an expensive proposal.

"It still is cheaper than standard gasoline and diesel fuel and very much cleaner," Summit Stage director John Jones said. "It's something that we could move into a little further down the road, maybe not tomorrow, but certainly in a few years."

In a road test last week, a bus running on compressed natural gas (CNG) conquered Fremont Pass 7 mph faster than a diesel bus, producing essentially zero emissions and with a slightly smaller engine than those of the Stage's existing bus fleet.

"It ran very well," Jones said. "It wasn't what we expected."

The buses may also need to be tested for performance in cold weather, but Jones said he doesn't expect lower temperatures to cause a problem.

The CNG buses are also less expensive to operate and are expected to have fewer mechanical problems than the filtering systems currently used to clean up the exhaust from the Stage's diesel fueled buses.

But making the switch to a CNG fleet would start with a $2 million up-front bill to build a new fuel station site where the gas would be compressed. The money would also help retrofit the county's garages with safety mechanisms that would be able to detect and automatically filter natural gas if there was a leak.

In small quantities, natural gas exposure would be a health concern for transit personnel. In large quantities, it could explode if an ignition was introduced.

The Summit Stage would also face the expense of replacing all of the existing diesel buses with coaches that run on natural gas.

The Stage might be eligible for grants to help cover the cost of a CNG fueling station, Jones said.

CNG buses, which cost approximately $40,000 each, are significantly cheaper than their $200,000 hybrid alternative.

Still, it would take the county 10 or 15 years to replace the entire Summit Stage fleet, through the natural retirement and replacement of about three buses per year, officials said.

The clean fuel source could also eventually be used to power other public vehicles including dump trucks and snowplows. Emergency vehicles will likely be confined to traditional gas and diesel, because they are often needed quickly and fueling up with natural gas takes a little longer, Jones said.